The present application describes systems and techniques relating to reference voltage circuits for differential analog-to-digital converters (ADCs), for example, flash ADCs used in image sensors. An ADC is an electronic circuit that measures an analog signal, which typically represents some real-world phenomenon (e.g., temperature, pressure, incident light, acceleration, speed), and converts it to a digital signal by comparing the analog signal to a known reference voltage.
FIG. 1 is a functional block diagram illustrating a prior art design 100 for providing a differential reference voltage for an ADC. The design 100 uses a resistive ladder 102 with a differential analog input reference that may be provided by voltage controlled current sources 104. The design includes circuits with either non-regulated or regulated high and low voltages VADCxe2x80x94HIGH, VADCxe2x80x94LOW set up around a common-mode voltage. The input references may be regulated by voltage regulators 106, 108, according to reference input voltages VREFxe2x80x94HIGH, VREFxe2x80x94LOW 
FIG. 2 is a schematic illustrating an example circuit 200 implementing the prior art design 100 shown in FIG. 1. In FIG. 2, an amplifier AMP, regulates the voltage VADCxe2x80x94HIGH according to a reference input VREFxe2x80x94HIGH. An amplifier AMP2 regulates the voltage VADCxe2x80x94LOW according to a reference input VREFxe2x80x94LOW. Resistors R1, R2, R3, R4 constitute a resistive ladder 202 for the ADC. A capacitor C3 is a differentially connected decoupling capacitor for the ladder, 202. Capacitors C1 and C2 are phase compensating capacitors for regulators 206, 208, respectively.
In one aspect, a reference voltage circuit for an analog-to-digital converter includes a resistive ladder having an even number of resistors and a midpoint between the resistors and a reference voltage source coupled to the midpoint. The resistive ladder may include four, eight, or more resistors. The reference voltage source may be a regulated voltage source that will accept both input and output currents. The reference voltage source may differ from a common-mode voltage of the analog input signal to the analog-to-digital converter by a small amount, such as by less than one half a least significant bit (xc2xd LSB) of the analog-to-digital converter.
The reference voltage circuit may also include a reference resistor coupled with the reference voltage source, a voltage-controlled current source coupled with the reference resistor, a voltage regulator coupled with the first voltage-controlled current source, and a low voltage source coupled to the voltage regulator. The voltage regulator may be an amplifier coupled in a feedback loop with the voltage controlled current source. The reference voltage circuit may also include a current mirror coupled with a power supply, the voltage regulator, and the resistive ladder, wherein the current mirror scales current in the resistive ladder by a factor of eight. Resistance matching may be provided between the reference resistor and a total combined series resistance of the resistive ladder.
In another aspect, an analog-to-digital converter may be manufactured, such as on a semiconductor substrate, to include a reference voltage circuit having a resistive ladder and a reference voltage source connected with a midpoint of the resistive ladder. The analog-to-digital converter may also include a plurality of comparators coupled with the resistive ladder, and a digital output encoder.
In another aspect, a method includes forming on a substrate, a resistive ladder having an even number of resistors and a midpoint between the resistors, and connecting a reference voltage source to the midpoint. The method may further include forming a reference resistor on the substrate, forming a first voltage-controlled current source on the substrate, forming a voltage regulator on the substrate, connecting the reference resistor with the reference voltage source, connecting the first voltage-controlled current source with the reference resistor, and connecting the voltage regulator with the first voltage-controlled current source and with a low voltage source. The method may also include connecting an amplifier in a feedback loop with the first voltage-controlled current source, where the amplifier is the voltage regulator.
In another aspect, a method includes applying a reference voltage to a midpoint of a resistive ladder in a circuit to create a plurality of reference voltages for an analog-td-digital converter. The method may further include providing a power source for the circuit, and providing a ground for the circuit. The method may also include providing an analog input to a plurality of comparators coupled with the resistive ladder to generate a digital output.